This invention relates to the production of continuous glass fibers, e.g., fibers made by melting particulate batch ingredients or minerals including basalt and the like, and, more particularly, to a method and apparatus for monitoring the average diameter of one or more of such fibers and for controlling the fiber drawing process to maintain the average diameter at a predetermined magnitude.
It is intended that the term "scattered," as used herein, denotes forward scattering, backscattering and right angle scattering of the incident radiation.
It is well known in the art that continuous glass fibers can be produced by attenuating a plurality of streams of molten glass into fibers, collecting the fibers into a strand and winding the strand into a package for subsequent use in manufacturing various products. The molten glass flows from a furnace and through a forehearth into a feeder or bushing which has a plurality of orifices formed therein. The molten glass flows from the orifices as streams which are pulled downwardly at a high rate of speed for attenuation into fibers. A plurality of the attenuated fibers are then gathered together into a strand, coated with a sizing and wound onto a collection tube on a winder collet.
The prior art has employed various systems to control the rotational speed of the winder collet in an attempt to maintain a uniform rate of attenuation and hence produce fibers which are closely similar in diameter and which have a uniform diameter throughout their length. The diameter of a package gradually increases as the strand is wound onto the collection tube; therefore, the rotational speed of the collet must be simultaneously decreased in order to maintain a constant rate of attenuation. Roberson, U.S. Pat. No. 3,265,476, discloses varying the winding speed at a programmed or patterned rate so that the pull speed or strand speed is approximately constant. An alternative method of controlling fiber diameter is disclosed in Roberson, U.S. Pat. No. 3,126,268, in which the set point temperature of the bushing is varied at a programmed or patterned rate to compensate for the increasing winding speed as the package is being built.
While such control systems give satisfactory results and represent a marked improvement over the control systems previously known in the art, we have now discovered that even more uniform fibers can be produced by utilizing means for monitoring the diameter of the fibers being produced and controlling one or more of the fiber forming factors, such as the set point temperature of the bushing, the winding speed of the collet or the environment beneath the bushing. If fibers having closely similar diameters and uniform diameters throughout their length were produced, the strand yardage per pound of glass supplied from the feeder, generally referred to as yardage, would be consistently uniform, thus promoting consumer reliance upon the product quality when strand or fiber diameter or weight per unit length is specified. In the prior art systems, generally, the pull speed or linear strand speed was not constant enough to provide fibers that have closely similar diameters and uniform diameters throughout their lengths. Even if such systems did accurately control the speed, one of the other myriad of interrelated factors that affect the fiber forming process may have changed, thus making the speed set by the controller improper and resulting in fibers that are not consistent with the desired specifications. For example, the molten glass head within the bushing must remain constant to achieve a constant flow rate through the bushing orifices. Also, the temperature of the molten glass must remain constant to provide a constant viscosity of the molten glass, and thus a constant flow rate through the orifices. Therefore, the actual diameter of the continuously drawn fibers at any point or points along their length must be measured while they are being drawn so that one or more of the fiber forming factors can be adjusted to obtain the desired diameter.
One difficulty encountered in measuring the diameter of glass fibers is that the diameters are often extremely small, generally on the order of a few micrometers. The weakness of glass fibers to abrasion makes the measurement of fiber diameters difficult, because any action to measure the fibers generally requires that the fibers be restrained or held in some fashion during the measurement. Therefore, contacting measurements do not lend themselves to the measurement of fiber diameters during the actual forming operation where the fibers are moving at extremely rapid rates which would cause abrasion damage.
Therefore, it is an object of this invention to provide a reliable and accurate method and apparatus for sensing the average fiber diameter of a plurality of fibers without contacting the fibers, such apparatus being responsive to small changes in the diameters of the fibers in order that the output of such apparatus can be used for measurement and/or control purposes.